Single-wall carbon nanotubes (SWNT), commonly known as “buckytubes,” have unique properties, including high strength, stiffness, thermal and electrical conductivity. SWNT are hollow, tubular fullerene molecules consisting essentially of sp2-hybridized carbon atoms typically arranged in hexagons and pentagons. Single-wall carbon nanotubes typically have diameters in the range of about 0.5 nanometers (nm) and about 3.5 nm, and lengths usually greater than about 50 nm. Background information on single-wall carbon nanotubes can be found in B. I. Yakobson and R. E. Smalley, American Scientist, Vol. 85, July-August, 1997, pp. 324–337 and Dresselhaus, et al., Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, Ch. 19. Multi-wall carbon nanotubes appear as multiple concentric shells of carbon nanotubes and have some properties similar to single-wall carbon nanotubes. Multi-wall carbon nanotubes generally have diameters greater than about 3.5 nm and have many more defects in their carbon walls.
Single-wall carbon nanotubes are generally made in high-temperature processes using a carbon source and a metallic catalyst, typically comprising Group VIb and/or Group VIIIb transition metals. Methods for synthesizing single-wall carbon nanotubes include DC arc processes, laser vaporization of graphite doped with transition metal atoms, high temperature, high pressure gas-phase syntheses involving a carbon-containing feedstock gas, such as carbon monoxide, and a volatile transition metal catalyst precursor, and chemical vapor deposition (CVD) processes in which single-wall carbon nanotubes are formed from a carbon-containing gas on nanometer-scale metal catalyst particles, which can be supported on a substrate or catalyst support.
The single-wall carbon nanotube product that results from these processes generally is in the form of black soot and, typically, the soot is fluffy or powdery. Fine and fluffy powders can easily become airborne and can often be difficult to handle in industrial processes.
In addition to single-wall carbon nanotubes, the products from these processes also comprise transition metal catalyst residues and carbon not in the form of single-wall carbon nanotubes, such as amorphous carbon, partially formed single-wall carbon nanotubes, and, in some cases, multi-wall carbon nanotubes. The SWNT-containing product from supported-catalyst CVD processes can also comprise a substrate or support material. The distribution of reaction products will vary depending on the particular process used and the operating conditions used in the process.
For many applications, a purified single-wall carbon nanotube product is preferred. Procedures to purify single-wall carbon nanotubes are directed at removing carbon not in the form of single-wall carbon nanotubes, metallic catalyst residues and, in the case of CVD processes, catalyst support material. Procedures for purifying single-wall carbon nanotubes often involve acids that react with the metal residues. Such procedures can be “wet” methods, such as those involving aqueous acid solutions, or “dry” methods that employ gaseous acids, e.g. HCl in vapor form. Examples of wet and dry methods for purifying single-wall carbon nanotubes are related in International Patent Publications “Process for Purifying Single-Wall Carbon Nanotubes and Compositions Thereof,” WO 02/064,869 published Aug. 8, 2002, and “Gas Phase Process for Purifying Single-Wall Carbon Nanotubes and Compositions Thereof,” WO 02/064,868 published Aug. 8, 2002, respectively, and included herein in their entirety by reference.
Although wet and dry purification procedures can yield purified single-wall carbon nanotubes, the resulting form of the single-wall carbon nanotubes can be difficult to handle in industrial processes. Similar to the form of as-produced single-wall carbon nanotubes, gas-phase purification techniques often result in a fluffy, fine powdery product. Wet purification procedures often involve formation of an aqueous slurry of single-wall carbon nanotubes. Separation of the purified single-wall carbon nanotubes from the aqueous phase, often done by filtration or centrifugation, is typically slow and tedious, and often results in matted, compressed and dense forms of single-wall carbon nanotubes, such as mats, papers and chunks, that are difficult to redisperse in subsequent processes.
An effective method for fast, efficient separation of single-wall carbon nanotubes from aqueous slurry is, therefore, desirable. Also needed is a purified form of single-wall carbon nanotubes that is easier to handle than fine powders.